The interior surfaces of pipelines employed for moving fluids tend to become encrusted through chemical reaction or deposits of solids from the fluids. As encrustation builds up, the maximum fluid carrying potential of the pipeline is decreased. It has long been a practice of pipeline operators to periodically clean the interior of pipelines by passing pipeline or cleaning pigs through them.
A cleaning pig is a device that fits in the interior of a pipeline and is moved by fluid flow through the pipeline. Some means is provided on the pig to engage the interior wall of the pipeline to scrape or brush the interior to dislodge solid materials. Pigs may be propelled with water, petroleum based fluids or gases such as compressed air or nitrogen. Some pigs are required to clean both linear pipelines and nonlinear piping connections such as elbows, 180 degree return bends and plug type headers.
One type of pig utilizes a cylindrical body and has helical or flange-like abrasive elements for scrubbing. When the pig travels around bends, the pig body gathers on its inner radius and stretches on its outer radius. This gathering effect causes the pig to clean the portion of pipeline near its inner radius more aggressively than the portion of the pipeline near its outer radius. Over time, use of this pig may cause premature pipe wear or failure.